As a sensor that is mounted in an electronic apparatus, a sensor that uses a charge detection circuit configured to output a voltage according to an electric charge output from a sensing element is known. For example, Patent Document 1 discloses a charge detection circuit in which a capacitor and a resistor connected in parallel are provided between an output terminal and an input terminal of an operational amplifier.
In a sensor using this charge detection circuit, in order to detect a signal having a comparatively low frequency (for example, about several mHz to several Hz), such as a biological signal, it is necessary to lower the cutoff frequency of a high-pass filter by a capacitor and a resistor. That is, it is necessary to increase the resistance value of the resistor (for example, to about 10 TΩ).
When increasing the resistance value of the resistor, the use of a typical resistive element is not practical in terms of circuit size. Accordingly, for example, Patent Document 2 discloses a configuration in which a weak inversion region of a MOSFET is used to form a resistor having a large resistance value.
However, one drawback when realizing a resistor using a weak inversion region of a MOSFET is that the resistance value fluctuates largely with fluctuation in process or temperature. For this reason, in realizing a resistor using a weak inversion region of a MOSFET, it is necessary to suppress the influence of fluctuation in process or temperature. For example, Patent Document 3 discloses a configuration in which the gate voltage of a MOSFET is adjusted using a plurality of current sources having temperature dependence, thereby suppressing temperature dependence of the drain current of the MOSFET.
Patent Document 1: Japanese Patent Application Laid-Open No. 2008-224230
Patent Document 2: International Publication No. 95/25349
Patent Document 3: PCT Japanese Translation Patent Publication No. 8-509312
In the configuration disclosed in Patent Document 3, in order to suppress temperature dependence of the drain current of the MOSFET, it is necessary to accurately set a plurality of parameters experimentally found, and adjustment is difficult.